The invention relates to an architecture for a hydraulic steering system, intended in particular for fitting to an aircraft.
Aircraft generally include nosewheel landing gear having one or more wheels that are steerable in order to enable the aircraft to be taxied. For aircraft of large size, one or more steerable bogies are sometimes provided on the main landing gear, in addition to the steering device for the nose landing gear.
The steerable portions of landing gear are generally actuated by one or more actuators fed by the pressure-generator device of the aircraft via a hydraulic steering block situated close to the actuators, as a general rule directly on the landing gear. In conventional manner, the hydraulic steering block comprises a directional-control valve, generally of the proportional type, serving to control the delivery of fluid to the actuator(s) so as to control the steering of the steerable portion of the landing gear in response to orders from the pilot.
Steering is generally not considered as being a function that is critical from the point of view of aircraft safety. Loss of steering does not lead to catastrophic consequences, and the steering function can be compensated by differential braking, optionally associated with differential thrust from the engines. If necessary, the aircraft can be towed.
It is therefore common practice for the hydraulic steering block to be connected to a single feed source only, the hydraulic block being arranged to allow the steerable portion of the landing gear to turn freely when the aircraft is stationary or in the event of the pressure-generator device not operating.
Nevertheless, the loss of the steering function can interfere severely with aircraft operation. Controlling an aircraft that is taxiing by differential braking does not enable it to make sharp turns, and for aircraft of large size that can be incompatible with the width available on taxiways. Furthermore, making sharp turns by blocking the wheels of the main landing gear on one side of the aircraft stresses said landing gear strongly in twisting which reduces its lifetime. In addition, requiring the use of a tractor to tow the aircraft can lead to a significant loss of time, and that can disturb the running of an airport in unacceptable manner.
In a conventional technique, the reliability of the steering function can be increased by duplicating the main feed circuit by means of an emergency feed circuit.
However, that solution when applied to the present situation presents numerous drawbacks. On large airliners, the hydraulic steering block of the nose landing gear is remote from the pressure-generator device of the aircraft (which is associated with the engines of the aircraft) by a distance of several tens of meters, and duplicating the pipework would give rise to harmful extra weight. Furthermore, segregation requirements make it essential for the main and emergency circuits to follow different paths through the structure of the aircraft, thereby complicating aircraft design.
The state of the art is also shown by documents: U.S. Pat. No. 4,422,920; U.S. Pat. No. 4,574,904; U.S. Pat. No. 4,190,130; and JP-A-59 109 495, and the teaching thereof is commented on below.
Document U.S. Pat. No. 4,422,290 describes an architecture for a steering system that comprises a steering control actuator connected to a proportional directional-control valve. In normal mode, a general selector connects the feed port of the directional-control valve to a pressure-generator device, and it connects the return port to a supply (which is not filled in normal mode), while in breakdown mode, said directional-control valve feed port is put into communication both with the pressure-generator device and with an accumulator. A genuine alternate mode of operation is not available.
Document U.S. Pat. No. 4,574,904 describes another architecture in which an accumulator is maintained under pressure in a normal mode of operation by an auxiliary pump. In breakdown mode, a valve puts the accumulator into communication with the central directional-control valve, said accumulator then no longer being connected to the auxiliary pump.
Document U.S. Pat. No. 4,190,130 describes another architecture having an auxiliary pump and an accumulator, in which the auxiliary pump draws the necessary fluid from the main supply.
Finally, document JP-A-59 109 495 describes a steering system architecture in which the emergency system possesses its own proportional directional-control valve, with the valve of the main circuit being shunted in breakdown mode.
The invention seeks to provide good reliability for the steering function of an aircraft without suffering the drawbacks or the limitations of the solutions mentioned above.
The hydraulic steering system architecture of the invention comprises at least one steering control actuator having chambers connected to the outlets of a directional-control valve which presents a feed port and a return port, and the architecture also comprises an accumulator and an electrically-driven pump unit associated with an emergency supply which is arranged to maintain a predetermined pressure level in the accumulator, the hydraulic system further comprising a general selector arranged, in a normal mode of operation, to connect the feed port of the directional-control valve to a pressure-generator device and the return port of the directional-control valve to a main supply associated with the pressure-generator device, while also ensuring that the emergency supply is filled, and in an alternate mode of operation, to connect the feed port of the directional-control valve to the accumulator.
Thus, while the pressure-generator device of the aircraft is functioning normally, the hydraulic steering block is fed as in the prior art by the pressure-generator device of the aircraft.
In the event of the pressure-generator device breaking down, which corresponds to an alternate mode of operation, the general selector switches over feed to the directional-control valve in such a manner that the valve is fed from the accumulator, with the electrically-driven pump unit then serving to reinflate the accumulator as fluid is consumed by the hydraulic steering block.
Thus, the steering function continues to be provided in the event of the pressure-generator device breaking down, and it is no longer necessary to have recourse to a heavy and expensive emergency feed circuit.
In a particular embodiment, the general selector comprises a normal mode valve and an alternate mode valve connected to the two inlets of a shuttle valve having an outlet connected to the feed port of the directional-control valve so that in normal operation mode, the normal mode valve connects the corresponding inlet of the shuttle valve to the pressure-generator device while the alternate mode valve connects the other inlet of the shuttle valve to the main supply, and in the alternate mode of operation, the normal mode valve connects the corresponding inlet of the shuttle valve to the main supply while the alternate mode valve connects the other inlet of the shuttle valve to the accumulator.
Advantageously, the general selector is arranged in the normal mode of operation to connect the return port of the directional-control valve to the main supply or to the emergency supply as a function of information from associated sensors, and in the alternate mode of operation to connect the return port of the directional-control valve to the emergency supply.
In which case, it is preferable for the return port of the directional-control valve to be connected both to the main supply and to the emergency supply, the general selector including a return valve which, in the normal mode of operation, leaves the connection to the main supply or to the emergency supply open, and in the alternate mode of operation, closes the connection to the main supply.
In an aspect of the invention, the accumulator is fitted with a pressure sensor for delivering information concerning the state of inflation of said accumulator, said information being used in the alternate mode of operation to control the electrically-driven pump unit to reinflate the accumulator.
Also advantageously, the accumulator is connected via a check valve to the pressure-generator device so that it can be filled and pre-loaded. The accumulator is also protected from excess pressure by a pressure-relief valve which is connected to the main supply, the pressure-relief valve being manually operable to enable the accumulator to be emptied for the purposes of maintaining the system.
Other characteristics and advantages of the invention appear more clearly in the light of the following description of a particular, non-limiting embodiment of the invention.